polymers

Review Pultrusion: A Review

Kirill Minchenkov, Alexander Vedernikov, Alexander Safonov * and Iskander Akhatov

Skolkovo Institute of Science and Technology, Center for Design, Manufacturing and Materials, 30/1 Bolshoi Boulevard, 121205 Moscow, Russia; [email protected] (K.M.); [email protected] (A.V.); [email protected] (I.A.) * Correspondence: [email protected]

Abstract: Pultrusion is one of the most efficient methods of producing polymer composite structures with a constant cross-section. Pultruded profiles are widely used in bridge construction, transporta- tion industry, energy sector, and civil and architectural engineering. However, in spite of the many advantages thermoplastic composites have over the thermoset ones, the thermoplastic pultrusion market demonstrates significantly lower production volumes as compared to those of the thermoset one. Examining the thermoplastic pultrusion processes, raw materials, mechanical properties of thermoplastic composites, process simulation techniques, patents, and applications of thermoplastic pultrusion, this overview aims to analyze the existing gap between thermoset and thermoplastic pultrusions in order to promote the development of the latter one. Therefore, observing thermoplastic pultrusion from a new perspective, we intend to identify current shortcomings and issues, and to propose future research and application directions.

Keywords: thermoplastic pultrusion; thermoplastic composites; fiber-reinforced materials

1. Introduction   Today, polymer composite materials have found wide application in various indus- tries [1,2]. This was made possible by extensive studies conducted over the last 70 years [3,4]. Citation: Minchenkov, K.; The popularity of composite materials results from their properties, such as high specific Vedernikov, A.; Safonov, A.; strength and stiffness [5–8]; improved durability [9,10]; high fatigue [11,12], chemical [13,14], Akhatov, I. Thermoplastic Pultrusion: and corrosion resistance [15–17]; and ease of transportation and assembly [18–20] of composite A Review. Polymers 2021, 13, 180. https://doi.org/10.3390/polym structures. Composite materials are produced by various processes, e.g., autoclave molding, 13020180 resin , , filament winding, and pultrusion [21,22]. Pultrusion is a process where a pack of reinforcement fibers impregnated by resin is pulled Received: 12 November 2020 through a heated block, where the process takes place [23]. This method Accepted: 30 December 2020 allows fabrication of products having constant cross-section [24,25]. The advantages of Published: 6 January 2021 pultrusion over other composite manufacturing processes are its high production rate of up to 5 m/min [1], higher efficiency [26,27] and low costs [28,29] of production, and the Publisher’s Note: MDPI stays neu- ability to produce profiles of virtually indefinite length [30]. There are thermoplastic and tral with regard to jurisdictional clai- thermoset matrix-based composites [31,32]. Thermosets are nonmelting polymers obtained ms in published maps and institutio- during chemical reaction (polymerization) between a resin and a hardener, while thermo- nal affiliations. composites can change their state and melt under heating. reinforcement is impregnated by hot melt thermoplastic polymer; then, after cooling, the part is ready for use. Compared to thermosetting composites, thermoplastic composites have higher impact toughness [33–36], are faster to produce [37,38], have higher service temperatures [39], Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. can be joined by welding [40–43], have less environmental impact [44–46], and can be recy- This article is an open access article cled [47–50]; their source materials have virtually unlimited shelf life [51–57]. Pultruded distributed under the terms and con- thermoplastic profiles are used in various structures and sectors, such as vehicles [58–63] ditions of the Creative Commons At- and aircrafts construction [64–66], aerospace [67–69] and civil engineering [70–73], en- tribution (CC BY) license (https:// ergy systems [74], restoration of deteriorated structures [75], marine applications [76–79], creativecommons.org/licenses/by/ oil and gas industries [80], electromagnetic interference shielding elements [81,82], window 4.0/). profiles [83], pipes [84,85], rebars [86,87] and rods [88–91].

Polymers 2021, 13, 180. https://doi.org/10.3390/polym13020180 https://www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 36

Polymers 2021, 13, 180 rated structures [75], marine applications [76–79], oil and gas industries [80], electromag-2 of 36 netic interference shielding elements [81,82], window profiles [83], pipes [84,85], rebars [86,87] and rods [88–91]. Today, the number of studies and publications in the field of thermoset pultrusion is Today, the number of studies and publications in the field of thermoset pultrusion is an an order of magnitude larger than the number of those in thermoplastic pultrusion, alt- order of magnitude larger than the number of those in thermoplastic pultrusion, although hough advantages offered by thermoplastic composites provide enough reason for deeper advantages offered by thermoplastic composites provide enough reason for deeper study. study. From industrial point of view, it is worth noticing that, for instance, Fiberline Com- From industrial point of view, it is worth noticing that, for instance, Fiberline Composites posites A/S, being one of the largest companies in the pultrusion manufacturing and the A/S, being one of the largest companies in the pultrusion manufacturing and the world’s world’sbiggest web-shopbiggest web-shop of fiberglass of profiles [ 92profiles], has numerous[92], has numerous types of pultruded types of pultruded structural profilesstructural available profiles foravailable purchase for withpurchase all of wi themth all being of them thermoset being ones.thermoset Several ones. questions Several questionsarise in this arise connection. in this connection. Why are there Why no are thermoplastic there no thermoplastic profiles available profiles for the available customers for theto purchase, customers despite to purchase, their numerous despite their advantages? numerous Why advantages? is there a Why well-developed is there a well-de- market velopedfor thermosetting market for profiles thermosetting and almost profiles no market and almost for thermoplastic no market fo ones?r thermoplastic ones? Currently, there is nono reviewreview on thermoplasticthermoplastic pultrusion. By analyzing the thermo- plastic pultrusion process from different perspectives (techno (technology,logy, raw materials, proper- proper- ties, numerical modeling, applications, etc.) an andd recalling the main publications regarding thermoplastic pultrusion (scientific (scientific articles, patents),patents), this article aims to understand why thermoplastic pultrusion failed to receive the attention and broad acceptance it deservesdeserves from thethe scientificscientific andand engineering engineering community, community as, as opposed opposed to thatto that of thermosetof thermoset pultrusion. pultru- sion.Exploring Exploring this issue, this issue, the authors the authors intend intend to attract to attract scientists’ scientists’ attention attention and stimulate and stimulate further furtherdevelopments developments in thermoplastic in thermoplastic pultrusion. pultrusi Sectionon. Chapter2 describes 2 describes the process the process of thermo- of ther- plasticmoplastic pultrusion pultrusion and and components components of pultrusionof pultrusion machines, machines, provides provides the the classification classification of ofthermoplastic thermoplastic pultrusion, pultrusion, analyzes analyzes basic basic parameters parameters of of pultrusion pultrusion process, process, andand reviewsreviews patents registeredregistered andand future future research research possibilities. possibilities. Section Chapter3 discusses 3 discusses raw materialsraw materials used usedin manufacturing in manufacturing of thermoplastic of thermoplastic composites, composites, mechanical mechanical properties properties of the of pultruded the pul- profiles,truded profiles, and promising and promising areas for areas the further for the investigations.further investigations. Section4 Chapter reviews 4 the reviews methods the methodsof process of modeling process modeling and discusses and discusses of possible of directions possible directions for scientific for work.scientific Section work.5 Chaptersanalyzes the5 analyzes possible the applications possible applications of thermoplastic of thermoplastic profiles, patents profiles, registered, patents andregistered, future andresearch future and research application and application possibilities. possibilities.

2. Thermoplastic Pultrusion and Process Parameters Luisier et al. were the firstfirst to proposepropose thethe classificationclassification ofof thermoplasticthermoplastic pultrusionpultrusion processes in two groups [[93].93]. The firstfirst is nonreactivenonreactive thermoplastic pultrusion where the process is based on the already polymerized materials, as opposed to the second one— reactive thermoplastic pultrusion where thermoplasticthermoplastic is polymerizedpolymerized duringduring chemicalchemical reaction betweenbetween thermoplasticthermoplastic resin resin and and catalyst/activator, catalyst/activator, with with simultaneous simultaneous impregna- impreg- nationtion of of fiber fiber reinforcement reinforcement (Figure (Figure1)[ 931) ].[93].

Figure 1. Thermoplastic pultrusion types.

A nonreactive pultrusion machine for thermoplastic composites consists of towpregs bobbins, guiding system, preheating chamber (preheater), heated forming die, cooling die, puller, and a cutting saw (Figure2)[ 30]. Pretreated fibers, blended intimately with

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Polymers 2021, 13, 180 3 of 36 A nonreactive pultrusion machine for thermoplastic composites consists of towpregs bobbins, guiding system, preheating chamber (preheater), heated forming die, cooling die, puller, and a cutting saw (Figure 2) [30]. Pretreated , blended intimately with thermoplasticsthermoplastics at at the the filament filament level level at at certain certain ratio, ratio, are are fed fed into into the the guiding guiding system system in orderin or- toder prevent to prevent their their entanglement entanglement and to and distribute to distribute fibers overfibers the over whole the sectionwhole section of the profile. of the Collimatedprofile. Collimated fibers are fibers then are fed then into fed a preheaterinto a preheater and heated and heated to a temperature to a temperature above above the meltingthe melting point point of the of thermoplasticthe thermoplastic in order in order to reduce to reduce the the time time the the reinforcement reinforcement stays stays in thein the heated heated die blockdie block and and to ensure to ensure uniform unif impregnationorm impregnation of reinforcement. of reinforcement. Various Various types oftypes heating of heating systems systems can be can used be inused the in process, the process, such such as convective as convective [94], [94], infrared infrared [95– [95–97], contact97], contact [98], and[98], microwave and microwave [99]. In [99]. practice, In practice, contact contact heating heating systems systems demonstrate demonstrate higher efficiency,higher efficiency, as compared as compared to convective to convective ones [ones94]. [94]. After After exiting exiting the preheatingthe preheating chamber, cham- uniformlyber, uniformly heated heated material material enters enters the heated the he dieated block die block where where the melting the melting of thermoplastic of thermo- takesplastic place takes and place the and profile the assumes profile assumes its final shape.its final In shape. order toIn accelerateorder to accelerate the consolidation the con- ofsolidation the polymer, of the thepolymer, formed the profile formed is profile fed into is thefed coolinginto the diecooling where die it where is cooled it is tocooled the near-ambientto the near-ambient temperature. temperature. At the lastAt the stage last the stage profile the profile is cut to is the cut required to the required lengths lengths with a flyingwith a saw flying [100 saw]. [100].

FigureFigure 2. Schematic 2. Schematic diagram diagram of the nonreactiveof the nonreactiv thermoplastice thermoplastic pultrusion pultrusion line. line.

NonreactiveNonreactive pultrusionpultrusion machinesmachines cancan alsoalso incorporateincorporate braidingbraiding appliancesappliances [[101,102].101,102]. BechtoldBechtold etet al. al. [ 94[94]] have have successfully successfully combined combined pultrusion pultrusion and and braiding, braiding, producing producing the pro- the fileprofile with with additional additional external external reinforcement. reinforcement. The Daimler The Daimler AG company AG company has investigated has investi- and patentedgated and the patented braided the pultrusion braided machine pultrusion allowing machine fabrication allowing of fabrication hollow profiles of hollow [103– 107pro-]. Memonfiles [103–107]. and Nakai Memon [108] and used Nakai the combination [108] used ofthe pultrusion combination and of braiding pultrusion to fabricate and braiding pipes reinforcedto fabricate with pipes jute reinforced fiber. They with tested jute variousfiber. They pulling tested speeds, various temperatures, pulling speeds, and tempera- pulling forces,tures, and and pulling also investigated forces, and thealso influence investigated of braiding the influence parameters of braiding such parameters as the braiding such angle,as the thebraiding gap between angle, the braiding gap between yarns, andbraiding the filling yarns, ratio. and the filling ratio. FromFrom thethe industrial point point of of view, view, it it is is worth worth noting noting the the knowledge knowledge that that was was devel- de- velopedoped in the in the leading leading countries countries with with patents patents on thermoplastic on thermoplastic pultrusion: pultrusion: USA, USA, China, China, Ger- Germany,many, and and France. France. Engineers Engineers developed developed various various techniques techniques to to control control the tension ofof thethe filamentsfilaments[ 109[109,110],,110], the the pressure pressure in thein diethe [111die, 112[111,112],], and the and size the of thesize die of cavity the die [113 cavity,114]. Different[113,114]. techniques Different techniques of fiber impregnation of fiber impreg [115nation–120] [115–120] and material and feeding,material suchfeeding, as sheet such feedingas sheet of feeding fibers andof fibers and thermoplastics [121] and [121] individual and individual fiber feeding fiber [122 feeding,123], [122,123], were de- veloped.were developed. Pultrusion Pultrusion is normally is normally used to create used profilesto create of profiles constant of cross-section;constant cross-section; however, engineers from Boeing and the Phillips Petroleum Company modified the mechanics of the however, engineers from Boeing and the Phillips Petroleum Company modified the me- process, making it possible to produce profiles of variable cross-section, either by using chanics of the process, making it possible to produce profiles of variable cross-section, multiple dies [124,125] or by modifying the die system [126–130]. either by using multiple dies [124,125] or by modifying the die system [126–130]. The combination of pultrusion and reaction injection molding (RIM) resulted in The combination of pultrusion and reaction injection molding (RIM) resulted in de- development of the RIM pultrusion (reactive pultrusion) process similar to the combination velopment of the RIM pultrusion (reactive pultrusion) process similar to the combination of thermoset pultrusion and injection molding [131–136], patented by Industrial Technology of thermoset pultrusion and injection molding [131–136], patented by Industrial Technol- Research Institute in 1993 [137]. The main difference between the reactive and nonreactive ogy Research Institute in 1993 [137]. The main difference between the reactive and nonre- pultrusion processes is the design of the heated die block. In the reactive pultrusion process, active pultrusion processes is the design of the heated die block. In the reactive pultrusion preheated unimpregnated fiber is fed into the heated die block where fiber impregnation process, preheated unimpregnated fiber is fed into the heated die block where fiber im- and polymerization of matrix take place (in situ polymerization), and the polymerized matrix has properties of thermoplastic melt [40]. The following polymers are typically used in the RIM pultrusion: (PC), (PE), (PU),

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Polymers 2021, 13, 180 pregnation and polymerization of matrix take place (in situ polymerization), and4 of 36the pol- ymerized matrix has properties of thermoplastic melt [40]. The following polymers are typically used in the RIM pultrusion: polycarbonates (PC), polyesters (PE), polyurethanes (PU),polymethylmethacrylates polymethylmethacrylates (PMMA), (PMMA), and and polyamides (PA) (in particular, (PA) (in PA-6 particular, synthesized PA-6 syn- thesizedfrom ε-caprolactam from ε-caprolactam (ε-CL) ) (ε-CL) monomer) [138]. Figure [138].3 shows Figure a schematic 3 shows illustration a schematic of the illustra- tionRIM of pultrusionthe RIM pultrusion die block [139 die]. block [139].

FigureFigure 3. Scheme 3. Scheme of the of reaction the reaction injection injection molding molding (RIM) pultrusion (RIM) pultrusion die block. die block. The important advantage of reactive pultrusion lies in the low viscosity of thermoplas- tic resinThe important solution as opposedadvantage to thermoplasticof reactive pultrusi polymers,on whichlies in improvesthe low andviscosity accelerates of thermo- plasticimpregnation resin solution and, in as turn, opposed increases to thermo productionplastic rate. polymers The important, which factor improves is the rateand ofacceler- atespolymerization, impregnation as and, it can in take turn, 1 toincreases 60 min for prod polymerizationuction rate. toThe complete, important depending factor is on the rate ofthe polymerization, temperature and as monomer-to-activator it can take 1 to 60 min ratio for [140 polymerization–142]. to complete, depending on the Furthertemperature in the chapterand monomer-to-activator we will discuss the parameters ratio [140–142]. of the nonreactive thermoplastic pultrusionFurther process, in the suchchapter as preheater we will temperature, discuss th temperaturee parameters and of geometry the nonreactive of the heated thermo- plasticdie, pressure pultrusion inside process, the heated such die, as coolingpreheate dier temperature, pulling temperature speed, andandpulling geometry of force, and their influence on the production process. We will not limit the discussion to the heated die, pressure inside the heated die, cooling die temperature, pulling speed, and the description of the process, but will also include a brief overview of articles investi- pullinggating force, a particular and their manufacturing influence on parameter the production from a scientific process. point We ofwill view, not and, limit finally, the discus- sionnote to promising the description areas for of future the process, research. but will also include a brief overview of articles investigating a particular manufacturing parameter from a scientific point of view, and, finally,2.1. Preheater note promising Temperature areas for future research. The aim of a preheating system analysis is to find the optimum temperature that 2.1.would Preheater allow Temperature maximum increase in production rate without compromising the performance of the profiles produced. Increase in pulling speed reduces the time a material stays in The aim of a preheating system analysis is to find the optimum temperature that the preheating chamber and, thus, requires the use of more efficient heating techniques. wouldIn 1997, allow maximum and Astrom increase [95] suggestedin production that a rate preheater without should compromising meet the following the perfor- mancerequirements: of the profiles the heating produced. should Increase be noncontact in pu (tolling prevent speed melting reduces of thermoplastic),the time a material con- stays intinuous the preheating (to prevent chamber overheating and, andthus, degradation requires the of theuse material), of more andefficient uniform heating (to prevent techniques. Intemperature 1997, Carlsson differences and Astrom in a material). [95] suggested that a preheater should meet the following requirements:However, the as washeating shown should in practice, be nonconta the use ofct a contact(to prevent preheater melting allows of engineers thermoplastic), to continuousspeed up the (to process,prevent increase overheating heating and efficiency, degradation and improve of the material), the shear and strength uniform of the (to pre- ventmaterial temperature [94]. The differences optimum preheater in a material). temperature is assumed to be close to the melting temperature of a thermoplastic, in spite of the fact that high preheating temperature reduces However, as was shown in practice, the use of a contact preheater allows engineers the viscosity and drag while reducing the probability of fiber breakage [143]. On the other tohand, speed preheating up the process, temperature increase that heating exceeds effi theciency, thermoplastic and improve melting the temperature shear strength may of the materialcause the [94]. partial The loss optimum of material preheater and increased temperat voidure content, is assumed especially to when be close using to contact the melting temperaturepreheaters. Inof addition,a thermoplastic, high preheat in spite temperatures of the fact result that in thehigh higher preheating surface temperature roughness re- ducesof a productthe viscosity [95]. and drag while reducing the probability of fiber breakage [143]. On the otherKerbiriou hand, andpreheating Friedrich temperature [144] experimentally that exceeds studied the basic thermoplastic manufacturing melting parameters, tempera- turenamely may temperaturecause the partial conditions loss onof material the preheater, and increased heated die void and coolingcontent, die, especially pressure when usingin the contact heated preheaters. die, and pulling In addition, speed, high and theirpreheat influence temperatures on density result and in mechanical the higher sur- properties. At the same time, Bechtold et al. [145] studied the effects of preheating, heated face roughness of a product [95]. die temperature, and pulling speed on the mechanical characteristics by using glass fiber– Kerbiriou and Friedrich [144] experimentally studied basic manufacturing parame- ters, namely temperature conditions on the preheater, heated die and cooling die, pressure in the heated die, and pulling speed, and their influence on density and mechanical prop- erties. At the same time, Bechtold et al. [145] studied the effects of preheating, heated die

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temperature, and pulling speed on the mechanical characteristics by using glass fiber– 6 ( 6) microbraided yarn. The influence of preheating temperature on the properties of produced profiles was determined by Evstatiev et al. [146].

2.2. Temperature and Geometry of the Heated Die The main component of the nonreactive pultrusion machine is the heated die block. The purpose of the die block is to melt the matrix, to impregnate fibers, and to impart a shape to the composite. To control the process of pultrusion, manufacturers equip the heated die block with thermocouples, pressure gauges, and electrical heaters [147]. On the one hand, the increase in temperature lowers the viscosity of the matrix, and increases pressure due to the thermal expansion, thus improving the impregnation of fibers [98]. As shown experimentally by Carlsson and Astrom [95], the increase in temperature results in better mechanical performance of the glass fiber and (GF/PP)-based composite. On the other hand, the maximum temperature is limited by the temperature of thermal degradation of polymers [148], which, if exceeded, can result in polymer burn- out and rejected products. High pressure and temperature may cause the fracture of rein- Polymers 2021, 13, 180 forcing fibers. Also, low viscosity in combination with low pulling speed and high 5pres- of 36 sure may force the matrix to move backward and accumulate at the entrance of the heated die [98]. Figure 4 shows the typical distribution of temperatures during the pultrusion polyamideprocess [149]. 6 (Nylon 6) microbraided yarn. The influence of preheating temperature on the propertiesExperimental of produced trials profiles with different was determined die geometries by Evstatiev were carried et al. [146 out]. by Michaeli and Jurss [147]. Evstatiev et al. explored the influence of heated die temperature on the prop- 2.2.erties Temperature of pultruded and profiles, Geometry using of the Heatedscanning Die electron microscopy, wide-angle X-ray scat- tering,The and main mechanical component testing of the [146]. nonreactive In the study pultrusion with jute machine fiber composites is the heated [108], die Memon block. Theet al. purpose observed of an the increase die block in flexural is to melt strength the matrix, at a certain to impregnate temperature; fibers, however, and to impartfurther aincrease shape toin thetemperature composite. resulted To control in a thereduction process thereof. of pultrusion, Schafer manufacturersand Gries [150] equipproposed the heatedthe unconventional die block with heating thermocouples, method for pressure braide gauges,d pultrusion and electrical process. heatersSimultaneously, [147]. On Os- the onewald hand, et al. [151] the increase analyzed in the temperature influence of lowers temperature the viscosity regime of on the the matrix, void content and increases of ther- pressuremoplastic due pultruded to the thermal profiles expansion, based on natural thus improving fibers, and the investigated impregnation the of influence fibers [98 of]. Asheating shown conditions experimentally on the byvoid Carlsson content. and Opti Astrommized [95 parameters], the increase of th ine temperaturepultrusion process results in(temperature better mechanical conditions performance in particular) of the were glass investigated fiber and polypropylene by Wongsriraksa (GF/PP)-based and Nakai composite.[152]. In [153], On the othereffects hand, of heating the maximum conditions temperature on the mechanical is limited performance by the temperature of carbon of thermalfiber reinforced degradation polymer of polymers (CFRP) [composites148], which, we if exceeded,re experimentally can result evaluated. in polymer Chen burn-out et al. and[154] rejected analyzed products. correlation High between pressure die and temper temperatureature and may properties cause the fracture(crystallinity, of reinforcing melting fibers.point, mechanical Also, low viscosity properties) in combination of the manufact withured low pullingprofiles. speed At the and same high time, pressure Lapointe may forceand Laberge the matrix Lebel to move[149] investigated backward and the accumulate use of a multi-die at the entrance system for of thethe heated better impreg- die [98]. Figurenation4 of shows thermoplastic the typical pultruded distribution rods. of temperatures during the pultrusion process [ 149].

Figure 4. Typical distribution ofof temperaturestemperatures insideinside thethe pultrusionpultrusion machine.machine.

ExperimentalAnother important trials parameter with different affecting die geometries the impregnation were carried of fibers out is by the Michaeli geometry and of Jurssthe heated [147]. die Evstatiev block [149], et al. the explored inner part the influence of which ofhas heated a tapered die temperaturesection linearly on thenarrowing proper- ties of pultruded profiles, using scanning electron microscopy, wide-angle X-ray scattering, and mechanical testing [146]. In the study with jute fiber composites [108], Memon et al. observed an increase in flexural strength at a certain temperature; however, further in- crease in temperature resulted in a reduction thereof. Schafer and Gries [150] proposed the unconventional heating method for braided pultrusion process. Simultaneously, Os- wald et al. [151] analyzed the influence of temperature regime on the void content of thermoplastic pultruded profiles based on natural fibers, and investigated the influence of heating conditions on the void content. Optimized parameters of the pultrusion process (temperature conditions in particular) were investigated by Wongsriraksa and Nakai [152]. In [153], the effects of heating conditions on the mechanical performance of carbon fiber reinforced polymer (CFRP) composites were experimentally evaluated. Chen et al. [154] analyzed correlation between die temperature and properties (crystallinity, melting point, mechanical properties) of the manufactured profiles. At the same time, Lapointe and Laberge Lebel [149] investigated the use of a multi-die system for the better impregnation of thermoplastic pultruded rods. Another important parameter affecting the impregnation of fibers is the geometry of the heated die block [149], the inner part of which has a tapered section linearly narrowing to the die exit (Figure5)[ 155,156]. Near the exit of the die block, the cross-section becomes Polymers 2021, 13, 180 6 of 36 Polymers 2020, 12, x FOR PEER REVIEW 6 of 36

to the die exit (Figure 5) [155,156]. Near the exit of the die block, the cross-section becomes constantconstant andand assumes assumes the the geometry geometry corresponding corresponding to the to desiredthe desired shape shape of a composite of a composite [157]. The[157]. tapered The tapered section section of the of die the block die block is described is described by the by angle the angle of taper of taper that that affects affects the pressurethe pressure and and backward backward motion motion of theof the thermoplastic thermoplastic melt. melt. In In order order to to minimize minimizefriction friction betweenbetween aa compositecomposite andand internalinternal surfacessurfaces ofof thethe diedie block, and to reduce the pulling force, thethe internalinternal surfacessurfaces ofof thethe diedie blockblock areare chromiumchromium platedplated [[158].158]. InIn additionaddition toto thethe taperedtapered diedie blockblock designsdesigns wherewhere thethe reinforcementreinforcement packpack isis shapedshaped andand impregnatedimpregnated byby wayway ofof pressurepressure exertedexerted uponupon aa materialmaterial byby internalinternal surfacessurfaces ofof thethe diedie block,block, therethere isis alsoalso aa diedie blockblock designdesign wherewhere thethe thermoplasticthermoplastic meltmelt isis forcedforced intointo fibersfibers byby specialspecial pinspins[ [97].97].

FigureFigure 5.5. AngleAngle ofof tapertaper ofof thethe heatedheated die.die.

2.3.2.3. Heated DieDie PressurePressure TheThe processprocess of of fiber fiber impregnation impregnation depends depends on theon the pressure. pressure. Pressure, Pressure, in turn, in dependsturn, de- onpends the on viscosity the viscosity of a polymer, of a polymer, pulling pulling speed, speed, and the and angle the angle of taper of taper [157]. [157]. Pressure Pressure in a diein a block die block originates originates from from thermal thermal expansion expansion of a polymer of a polymer inside inside a tapered a tapered die block die [block147]. It[147]. is very It is difficultvery difficult to evaluate to evaluate the influencethe influence of pressureof pressure on on the the quality quality of of a a compositecomposite experimentally,experimentally, asas highhigh pressurepressure valuesvalues cancan onlyonly bebe achievedachieved atat highhigh pullingpulling speedspeed thatthat adversely affects the quality of material because of resulting high void content [147]. adversely affects the quality of material because of resulting high void content [147]. Fa- Fanucci et al. [159] manufactured special sensors and studied their application for pressure nucci et al. [159] manufactured special sensors and studied their application for pressure registration during thermoplastic pultrusion. registration during thermoplastic pultrusion. 2.4. Temperature of a Cooling Die 2.4. Temperature of a Cooling Die The profile exiting the die block can lose its shape under external forces due to plasticityThe profile of the polymer exiting the at high die block temperatures. can lose it Tos shape prevent under the loss external of shape, forces it isdue necessary to plas- toticity cool of the the profilepolymer below at high the temperatures. glass transition To temperatureprevent the loss [147 of]. shape, As the it profile is necessary already to hascool the the desiredprofile below shape the at theglass cooling transition stage, temperature the cooling [147]. die hasAs the a constant profile already cross-section. has the Indesired order shape to achieve at the a cooling sharp temperaturestage, the cooling gradient, die has the a distanceconstant betweencross-section. the heated In order and to coolingachieve diesa sharp is rather temperature small [157 gradient,]. The experiments the distance by between Carlsson the et al.heated [95] andand by cooling Kerbiriou dies etis al.rather [144 small] show [157]. that The cooling experiments temperature by Carlsson influences et theal. [95] surface and roughnessby Kerbiriou of aet product, al. [144] andshow its that flexural cooling and temperature shear strength. influences the surface roughness of a product, and its flex- ural Astromand shear et al.strength. [143] experimentally investigated the influence of process parameters in general,Astrom and ofet theal. [143] cooling experimentally die in particular, investig on theated degree the influence of crystallinity, of process and, parameters therefore, onin general, the mechanical and of the properties cooling ofdie thermoplastic in particular, composites.on the degree More of crystallinity, recently, Michaeli and, there- and Blaurockfore, on the [160 mechanical] discussed properties the relationship of ther betweenmoplastic cooling composites. zone parametersMore recently, and Michaeli surface qualityand Blaurock of produced [160] discussed profiles. the Ghaedsharaf relationship et between al. [161] studiedcooling zone the effects parameters of cooling and sur- die temperatureface quality andof produced pulling speed profiles. on theGhaedsharaf resin impregnation, et al. [161] void studied content, the andeffects quality of cooling of the finaldie temperature surface. and pulling speed on the resin impregnation, void content, and quality of the final surface. 2.5. Pulling Speed 2.5. PullingThe most Speed important pultrusion parameter affecting all other parameters is the pulling speed.The The most pulling important speed determinespultrusion parameter the time the affecting reinforcement all other and parameters a polymer is stay the pulling within thespeed. preheater The pulling and inside speed a diedetermines block. Impregnation, the time the pressure reinforcement within theand heated a polymer die block, stay pullingwithin the force, preheater heating and uniformity, inside a anddie block. viscosity Impregnation, of thermoplastic pressure melt—all within depend the heated on thedie pullingblock, pulling speed [force,98]. It heating was experimentally uniformity, and established viscosity thatof thermoplastic reduction in melt—all flexural strength depend ison associated the pulling with speed increase [98]. It in was a pulling experimental speed [ly162 established,163]. In addition, that reduction the increase in flexural in a pullingstrength speed is associated can adversely with increase affect the in shear a pulling strength speed and [162,163]. interlaminar In addition, shear strength the increase [162].

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Carlsson and Astrom [95] observed formation of defects at specimen surfaces with the increase in pulling speed. They attributed it to matrix sticking to the internal surfaces of the heated die block due to high pulling speed and high cooling temperature. Wiedmer and Manolesos also observed the shift from glossy to rough surfaces [98]. The relationships between pulling speed and compressive, flexural, and interlaminar shear strength of thermoplastic pultruded composites were experimentally analyzed by Astroem et al. [164]. Cho et al. [139] investigated the influence of pulling speed, heating temperature, and the reinforcement volume fraction on the temperature evolution of the resin, its conversion, and physical and mechanical characteristics. At the same time, aim- ing to achieve higher pulling speeds, Squires et al. conducted an experimental study by varying heating and cooling temperatures, as well as pressure profiles [165]. Azari [166] investigated the influence of pulling speed on the wet-out and mechanical properties of pultruded strands. Seeking to optimize pulling speed, Ozturk et al. explored the sensitivity of the process to changes in the pulling speed by changing the manufacturing parameters of the pultrusion line [167]. Subsequently, the effects of pulling speed on the microstruc- tural and mechanical characteristics of pultruded profiles were investigated by Evstatiev et al. [146]. Nunes et al. [168] analyzed the influence of pulling speed and heating temper- ature on the mechanical and physical properties of pultruded profiles manufactured of towpregs. The effect of pulling speed on the mechanical performance of CFRP composites was evaluated by Wongsriraksa and Nakai [153]. Pulling speed optimization in the case of glass-fiber-reinforced polyamide-6 (PA-6) composite manufactured by thermoplastic reaction injection pultrusion technique is discussed in [154]. Simultaneously, Lapointe and Laberge Lebel investigated effects of pulling speed on the void content and quality of impregnation [149].

2.6. Pulling Force Pulling force can change depending on the pulling speed, section geometry, taper angle, and viscosity of a polymer. The critical value of pulling speed should be tightly controlled in order to prevent production interruptions and to maintain the integrity of a profile [169]. As shown by Carlsson and Astrom [95], the pulling speed is the main factor affecting the pulling force. Astrom [155] succeeded in establishing the relation between the taper angle and pulling force. At angles exceeding 5◦, the pulling force is relatively low; the drastic increase in pulling force is observed with the decrease in the angle of taper. In addition, the increase in the perimeter of a profile cross-section also results in the increased pulling force. The correlation between pulling force and pulling speed was experimentally investigated by Nakai and Morino [170].

2.7. Future Trends Analysis of the thermoplastic pultrusion process and its parameters, as well as consid- eration of thermosetting pultrusion scientific and industrial state-of-the-art, demonstrate that deeper research is needed to better understand the peculiarities of the thermoplastic pultrusion process. The deeper knowledge of the thermoplastic pultrusion process will stimulate the interest in this manufacturing technique from the scientific and engineering community. This subchapter briefly discusses promising directions for future investigations in this field. All the topics listed below require careful research, since there are currently few publications available on the mentioned subject, or research has not been conducted at all. Although some relationships between process parameters and mechanical characteris- tics of the thermoplastic pultruded products have been established, extensive experimental research is needed to understand the direct influence of these parameters on each of the following mechanical properties at different loading conditions and strain rates: tension, compression, flexure, buckling, shear, creep, and fatigue. Degree of crystallinity, melting, and consolidation evolution during polymerization depends on the temperatures used; thus, a deeper understanding of this interconnection is necessary in order to improve Polymers 2021, 13, 180 8 of 36

process outcomes. The influence of the die geometry defining the thickness of the manufac- tured profiles and, thus, determining mechanical properties and shape distortions of the final product, also requires detailed research. Moreover, severity of process-induced shape distortions both in thermoset [12,23,171] and in thermoplastic pultrusion depends on the temperature and, therefore, is another potential field of investigation. The relationship between process parameters and formation of voids, cracks, and delaminations also require in-depth research. A successful application of thermoplastic pultruded structures in harsh and severe environments will require better understanding of the influence of process parameters on the service life of the produced profiles. Finally, in order to avoid expensive trial–error experiments when studying the influ- ence of process parameters of the thermoplastic pultrusion, we need better optimization and numerical simulation algorithms. Moreover, the existing models require refinement to improve control over process parameters and to obtain better outcomes of the thermoplastic pultrusion process. The studies discussed above mostly deal with profiles of simple cross-sections (rods, flat profiles). Currently, there is a lack of studies describing the thermoplastic pultrusion of complex shape profiles, such as pipes, channels, I-beams, decks, etc., commonly used in the construction industry. Despite the large number of publications on pultrusion with unidirectional reinforcement, there are no studies on thermoplastic pultrusion with various reinforcement types, and on application of fabrics, mats, and veils. Moreover, there are no studies on the stability of the thermoplastic pultrusion process; i.e., how many profiles (particularly of complex shape) of steady, acceptable quality can be produced within a single manufacturing cycle. Furthermore, successful scaling, development, and industrial application of thermoplastic pultrusion will require more studies on manufacturing process controls and elimination of defects.

3. Raw Materials and Properties of Obtained Composites Final properties of a material depend both on the quality of manufacturing and on the quality of raw materials. The main problem in thermoplastic composite manufacturing is the need to ensure good impregnation of reinforcing fibers with matrix, as the viscosity of thermoplastic polymers is significantly higher than that of the thermosetting ones, e.g., the average viscosity of thermosetting polymers is 0.03–1 Pa·s [142], as opposed to 500–5000 Pa·s [172] in case of thermoplastic ones. One way to simplify the process of nonreactive thermoplastic pultrusion is the use of prepregs where reinforcing fibers are in the close contact with matrix uniformly distributed over the whole length of a prepreg. When the prepreg enters the die block, thermoplastic polymer contained in the prepreg will melt and impregnate the fibers under pressure. Tables1 and2 show properties of some polymers and fibers used in the thermoplastic pultrusion. Longmuir and Wilcox proposed a novel technique allowing a variable number of fiber strands to be used during the manufacturing process [173]. Thomasset et al. performed a rheological study on the polypropylene and long-glass-fiber composites manufactured by pultrusion [174]. Simultaneously, Broyles et al. [175–177] studied the influence of fiber siz- ing agents on the mechanical properties and moisture absorption. Next, Roy et al. [178,179] succeeded in improving compression behavior of pultruded composites by modifying ma- terial composition and parameters of the thermoplastic pultrusion process. Subsequently, Fink and Ganster [180] conducted an experimental study of the influence of synthetic fibers and of the choice of matrix on mechanical properties of composites. A novel tool intended for the manufacturing of thermoplastic pultruded profiles was proposed by Novo et al. [181]. Tao et al. [182] analyzed mechanical performance, thermal stability, and mor- phology of composites based on long- and short-glass-fiber reinforcements. The influence of fiber content on mechanical and tribological properties, morphology, and thermal stabil- ity of pultruded (POM)–basalt fiber composites was studied by Wang et al. [183]. Kahl et al. [184] used different types of reinforcement (cellulose and glass fibers) and matrix material (polypropylene and polyamide) to evaluate the influence of raw Polymers 2021, 13, 180 9 of 36

materials on the mechanical performance of manufactured specimens. Shayan Asenjan et al. [185] conducted the experimental study to understand a correlation between the length of fibers and high-velocity impact performance. Chen et al. [154] investigated the influence of volume fraction of reinforcement on the density, heat distortion temperature, void occurrence, and mechanical characteristics of the glass fiber–polyamide-6 (PA-6) com- posites produced by RIM pultrusion. Recently, the relationship between impregnation and mechanical properties was studied by Saito et al. [186]. At the same time, seeking for a reduction in carbon footprint, Asensio et al. [187] studied the possibility to use recycled material for the pultrusion of thermoplastic composites.

Table 1. Polymers used as a matrix in thermoplastic pultrusion.

Elastic Tensile Flexural Flexural Melting Glass Transition Density, Material Modulus, Strength, Modulus, Strength, Reference Point, ◦C Temperature, ◦C g/cm3 GPa MPa GPa MPa PBT 230–223 31–60 1.21–1.38 1.8–2.5 40–55 1.9–2.8 76 [188–191] PA 6 220 49–75 1.10–1.12 2.8 48–80 1.9–3.2 69–117 [189,191,192] PA 66 268 60–70 1.06–1.12 2.8–3.9 30–85 1.2–3.7 86 [189,191,193,194] PA 12 174–185 55 1.01–1.03 0.5–1.9 45–70 0.36–1.2 - [191] PP 160–175 −15–−8 0.89–0.92 1.0–2.0 28–41 0.8–1.7 45–55 [169,188,189,191–193,195] PEEK 334–345 143–158 1.29–1.34 3.1–8.3 90–115 2.8–3.9 110 [149,169,188,191,192,196] PEKK 360 154–171 1.27–1.31 4.0 110 - - [188,189,191] PET 243–250 60–88 1.30–1.38 2.5–4.0 50–70 2.8 110 [188,189,191,193] PEI 216–220 209–249 1.26–1.70 2.7–6.4 100–105 2.9–3.3 151 [188,189,191,196] PES 220–238 220–246 1.36–1.58 2.4–8.6 83–126 [188,191] PMAA 105–160 82–105 1.17–1.26 2.8–3.4 62 3.2 97 [191,193] PPS 280–290 74–92 1.35–1.43 3.4–4.3 66–93 3.4–4.1 96–151 [188,189,191,192,196,197] PLA 150–162 55–75 1.18–1.26 0.5–3.5 21–170 1.8–2.8 - [189,191,195] HDPE 130–137 −133–−118 0.95–0.97 0.7–1.4 20–40 1.2 - [189,191,195,198] LDPE 105–125 −133–−103 0.92–0.93 0.1–0.4 5–17 - - [189,191] PC 255–267 −158–−134 1.18–1.22 2.4 55–75 2.1–2.4 80–93 [189–193] PE 104–113 −133–−59 0.92 0.2 10–18 - - [188,191] PU 220–230 −60–−19 1.15–1.25 0.1–0.7 5–28 - - [188]

Table 2. Fibers used as reinforcement in thermoplastic pultrusion.

Tensile Tensile Density, Poisson’s Material Modulus, Strength, Reference g/cm3 Ratio GPa GPa E-Glass 2.5–2.54 70–73 1.5–2.3 0.20–0.30 [1,188,189,199] S-Glass 2.46 90 4.5 0.21–0.23 [1,188] Carbon 1.94–2.15 585–725 2.2–3.8 0.25–0.30 [1,188] Flax fibers 1.5 50 0.5–0.9 - [1,189] Jute fibers 1.3 26.5 0.4–0.7 - [200] Hemp fibers 1.45 64 0.69 - [189] Graphite 1.90 3.3 - 0.28 [197] Aramid 1.45 125 2.8–3.5 0.35 [188,199]

Various additives (fillers), with the most popular being nanotubes, can improve the performance of composites. Nanotubes improve interlaminar shear strength, interfacial shear strength, and delamination resistance of a composite [201,202]. Addition of Ni powders increases the flexural modulus; the optimal ratio of matrix, filler, and Ni powder improves the mechanical performance of composites [81]. Various fiber coatings make it possible to improve tensile, compression, and flexural strength of a composite with a 2% increase in material weight cost [175]. Markov [203] showed how the distribution of filler particles within the pultruded composites affects their electric characteristics. Recently, Chen et al. [141] experimentally analyzed the influence of activators and initiators on the polymerization process. Several prepreg types for thermoplastic pultrusion are currently available on the market: preconsolidated tapes (Figure6a), commingled yarns (Figure6b), and towpregs Polymers 2020, 12, x FOR PEER REVIEW 10 of 36

improves the mechanical performance of composites [81]. Various fiber coatings make it possible to improve tensile, compression, and flexural strength of a composite with a 2% increase in material weight cost [175]. Markov [203] showed how the distribution of particles within the pultruded composites affects their electric characteristics. Recently, Chen et al. [141] experimentally analyzed the influence of activators and initiators on the polymerization process. Polymers 2021, 13, 180 Several prepreg types for thermoplastic pultrusion are currently available 10on of the 36 market: preconsolidated tapes (Figure 6a), commingled yarns (Figure 6b), and towpregs (Figure 6c) [192,204]. Seeking to optimize thermoplastic pultrusion process, Iftekhar [205] explored the influence of fillers and additives on the viscosity of resins. The relationship (Figurebetween6c) width/thickness [ 192,204]. Seeking of the to optimize prepregs thermoplastic and the mechanical pultrusion and process, physical Iftekhar properties [205 of] exploredcomposites the was influence studied of fillersby Mariatti and additives [206]. Hedayati on the viscosityVelis et al. of analyzed resins. The the relationship influence of betweenpolymer width/thickness matrix and of a ofseries the prepregsof prepregs and on the the mechanical mechanical and properties physical propertiesof pultruded of compositescomposites was[198]. studied by Mariatti [206]. Hedayati Velis et al. analyzed the influence of polymer matrix and of a series of prepregs on the mechanical properties of pultruded composites [198].

(c) (a) (b) FigureFigure 6. 6.Prepregs Prepregs schemes: schemes: ( a(a)) preconsolidated preconsolidated tapes, tapes, ( b(b)) commingled commingled yarns, yarns, and and ( c()c) towpregs. towpregs.

3.1.3.1. Preconsolidated Preconsolidated Tape Tape PreconsolidatedPreconsolidated tape tape (PCT) (PCT) consists consists of of reinforcement reinforcement fibers fibers impregnated impregnated with with a a ther- ther- moplasticmoplastic polymerpolymer atat aa specificspecific volume fraction. The The PCT PCT fabrication fabrication method method is is similar similar to tothat that of of pultrusion—hot pultrusion—hot thermoplastic thermoplastic melt melt is injected is injected into into the theheated heated die dieblock block [192]. [192 The]. Thematerial material is then is then cooled cooled and and wound wound onto onto reels reels for for storage storage and transport.transport. FigureFigure7 7shows shows thethe schematic schematic illustration illustration of of a a PCT PCT production production machine machine [ 192[192].].

FigureFigure 7. 7.Schematic Schematic illustration illustration of of a a preconsolidated preconsolidated tape tape (PCT) (PCT) production production machine. machine.

Currently,Currently, producedproduced PCT can can have have widths widths of ofup up to 300 to 300mm mmand thicknesses and thicknesses of 0.125– of 0.125–0.5000.500 mm [188]. mm [ 188The]. most The mostpopular popular PCTs PCTs are produced are produced with withfiber fiber volume volume fraction fraction of 60%, of 60%,at the at rate the rateof 20–60 of 20–60 m/min. m/min. PCT PCTcan canbe produced be produced in a in towpreg a towpreg production production line line with with the theadditional additional heated heated die die installed installed at the at the end end of ofthe the line line [194,207]. [194,207 ].

3.2.3.2. Commingled Commingled Yarns Yarns (CY) (CY) CommingledCommingled yarnsyarns (CY)(CY) are composed composed of of in intermingledtermingled matrix matrix and and reinforcement reinforcement fil- filamentsaments [208]. [208]. One One of of the the ways ways to to manufactur manufacturee CY isis aa mixingmixing ofof fibersfibers during during winding winding with the use of a winding machine (Figure8)[ 188]. In CY production it is possible to ensure uniform distribution of matrix and reinforcement filaments over the whole length of prepreg while maintaining the desired volume fraction of reinforcing material [99]. Distribution of filaments plays a very important role, as it affects flexural performance of a composite, its specific weight, and fiber volume fraction in a composite. There are four types of mixed fibers prepregs currently available on the market: commingled, cowrapped, core- spun, and stretch-broken yarns [172]. The most popular are commingled yarns [209–213]. Under pressure from thermoplastic melt, reinforcing fibers tend to aggregate during the impregnation and form agglomerations (Figure 10) [213]. Polymers 2020, 12, x FOR PEER REVIEW 11 of 36

Polymers 2020, 12with, x FOR the PEER use REVIEW of a winding machine (Figure 8) [188]. In CY production it is possible to en- 11 of 36 sure uniform distribution of matrix and reinforcement filaments over the whole length of prepreg while maintainingwith the use ofthe a desiredwinding volumemachine fraction (Figure 8)of [188].reinforcing In CY productionmaterial [99]. it is Dis- possible to en- sure uniform distribution of matrix and reinforcement filaments over the whole length of tribution of filaments plays a very important role, as it affects flexural performance of a prepreg while maintaining the desired volume fraction of reinforcing material [99]. Dis- composite, its specific weight, and fiber volume fraction in a composite. There are four tribution of filaments plays a very important role, as it affects flexural performance of a types of mixed fibers prepregs currently available on the market: commingled, cow- composite, its specific weight, and fiber volume fraction in a composite. There are four Polymers 2021, 13, 180 rapped, core-spun,types and of mixedstretch-broken fibers prepregs yarns [172].currently The available most popular on the are market: commingled commingled,11 of 36 cow- yarns [209–213].rapped, Under core-spun, pressure fromand stretch-brokenthermoplastic yarnmelt,s [172].reinforcing The most fibers popular tend to are ag- commingled gregate during yarnsthe impregnation [209–213]. Under and fopressurerm agglomerations from thermopl (Figureastic melt, 10) [213]. reinforcing fibers tend to ag- gregate during the impregnation and form agglomerations (Figure 10) [213].

FigureFigure 8. 8. CommingledCommingled yarn yarn fabrication. fabrication. Figure 8. Commingled yarn fabrication. 3.3.3.3. Towpregs Towpregs 3.3. Towpregs TowpregsTowpregs fabrication consistsconsists inin mixing mixing fine-powdered fine-powdered thermoplastic thermoplastic polymer polymer with with re- inforcing fibers. In dryTowpregs methods fabrication of towpregs consists fabrication, in mixing reinforcing fine-powdered fibers passthermoplastic through thepolymer with reinforcing fibers. In dry methods of towpregs fabrication, reinforcing fibers pass through chamber with powderedreinforcing polymer fibers. In anddry aremethods further of fedtowpregs into the fabrication, heated chamber reinforcing where fibers ther- pass through the chamber with powdered polymer and are further fed into the heated chamber where moplastic polymerthe chamber is ultimately withjoined powdered with polymer reinforcement and are fibers. further In fed 2000, into a pultrusion the heated headchamber where thermoplastic polymer is ultimately joined with reinforcement fibers. In 2000, a pultrusion for producing towpregsthermoplastic materials polymer was is ultimately patented at joined the University with reinforcement of Minho fibers. [214]. In By 2000, con- a pultrusion head for producing towpregs materials was patented at the University of Minho [214]. By vention, towpreghead machines for producing consist towpregs of five components: materials was fiber pate creel,nted guidingat the University system, powderof Minho [214]. By convention, towpreg machines consist of five components: fiber creel, guiding system, feeder, heatingconvention, chamber, andtowpreg winding machines mechanism consist (Figureof five 9components:)[ 192]. The fiber powder creel, feeder guiding system, powder feeder, heating chamber, and winding mechanism (Figure 9) [192]. The powder can utilize variouspowder powder feeder, agitation heating chamber, systems, and such winding as pneumatic mechanism [194 ],(Figure vibration 9) [192]. [192 The], powder feeder can utilizefeeder various can utilizepowder various agitation powder systems, agitation such systems, as pneumatic such as [194],pneumatic vibration [194], vibration and electrostatic [188]. To handle the problem of high viscosity of thermoplastic melts, [192], and electrostatic[192], and [188]. electrostatic To handle [188]. th eTo problem handle thofe highproblem viscosity of high of viscositythermoplastic of thermoplastic the alternative method of wet fabrication can be applied, where a solution of thermoplastic melts, the alternativemelts, themethod alternative of wet method fabrication of wet can fabrication be applied, can where be applied, a solution where of a ther-solution of ther- polymer in a solvent is used for impregnation. Reinforcement fibers are impregnated with moplastic polymermoplastic in a solvent polymer is used in a solvent for impregnation. is used for impregnation. Reinforcement Reinforcement fibers are impreg- fibers are impreg- a solution of thermoplastic polymer and then placed into a heated chamber to evaporate nated with a solution of thermoplastic polymer and then placed into a heated chamber to natedsolvent, with leaving a solution the neat of thermoplastic thermoplastic polyme polymerr and on fibers. then placed However, into as a heated the solvent chamber is quite to evaporate solvent, leaving the neat thermoplastic polymer on fibers. However, as the sol- evaporatedifficult to solvent, remove leaving completely, the neat this canthermoplastic result in increased polymer porosity on fibers. of However, the finished as compos-the sol- vent is quite difficult to remove completely, this can result in increased porosity of the ventite [188 is ].quite Optimization difficult to of remove the towpregs completely, manufacturing this can processresult in by increased means of porosity Taguchi’s of DOE the finished compositefinished [188]. composite Optimization [188]. Optimizationof the towpregs of the manufacturing towpregs manufacturing process by meansprocess by means (design of experiments)of Taguchi’s method DOE (design was performed of experiments) by Novo me etthod al. was [215 ].performed Nunes et by al. Novo [168] et al. [215]. ofstudied Taguchi’s the influence DOE (design of pulling of experiments) speed and furnacemethod temperature was performed on the by polymer Novo et content al. [215]. in Nunes et al. [168]Nunes studied et al. the [168] influence studied ofthe pulling influence speed of pulling and furnace speed andtemperature furnace temperature on the on the towpregs. polymer content in towpregs. polymer content in towpregs.

Figure 9. SchematicFigure illustration 9. Schematic of a illustration towpreg production of a towpreg machine. production machine. Figure 9. Schematic illustration of a towpreg production machine. 3.4. Mechanical Properties of Obtained Composites Typically, the same standards are used in mechanical testing of thermoplastic and thermoset pultrusion samples. For instance, ASTM D6641-16 is used for compression [216], ISO 527-5 is used for tension [217], ASTM D790-15e2 is used for flexure [218], ASTM D7078/ 7078M-12 is used for in-plane shear [219], and ASTM D2344-16 is used for interlaminar shear testing [220]. For comparison purposes, we have listed some mechanical properties of pultruded thermoplastic (Table3) and thermoset (Table4)[1,30] matrix composites. Polymers 2021, 13, 180 12 of 36

Table 3. Mechanical properties of thermoplastic pultruded components.

Volume Pultrusion Flexural Flexural Tensile Elastic Modulus, Notched Izod Impact Interlaminar Shear Material Reference Fraction Speed, m/min Strength, MPa Modulus, GPa Strength, MPa GPa Strength, J/m2 Strength, MPa GF/Nylon 12 0.50 0.3–3.0 380–610 - - - - 15–40 [162] GF/Nylon 6 0.71–0.75 0.1–0.9 359–469 - 828–869 - 1868–2348 - [199] GF/ABS 0.75 0.1–0.9 538 - 710 - - - [199] GF/PPS 0.70–0.75 0.1–0.9 965 - 793 - - - [199] GF/PMMA 0.75 0.1–0.9 656–863 - 897–1035 - 1815–2188 - [199] GF/PBT 0.38–0.41 0.1–1.2 ------[144] GF/PP 0.35 0.01–1.5 465–485 22–24 - - - - [94,95] GF/PP 0.32 0.2–0.3 299–359 15–18 302–409 20–23 - 27–28 [192] GF/PP 0.37 0.2–0.3 571–620 24–28 516–597 24–26 - 25–28 [192] GF/PP 0.53–0.59 0.9 113–121 22 279–331 27–33 - - [194] GF/PP 0.52 0.2–0.3 146–170 27–29 314–358 32–35 - 7–8 [192] GF/PMMA - 0.4 414 - 720 - 2400 - [221] GF/PMMA - 0.7 207 - 530 - 1300 - [221] GF/PMMA - 1.0 100 - 410 - 700 - [221] GF/PA 6 * 0.70 0.8 800–1060 26–34 - - - 61–70 [154] GF/PU * 0.50 2.7 210 6 - - - - [222] CF/PEEK 0.55 0.06–0.6 1150–1380 108–130 - - - - [143] CF/Nylon 6 0.57 0.1–0.9 498 - 1496 - 1708 - [199] CF/PPS 0.58 0.1–0.9 1365 - 1172 - 1601 - [199] CF/PP 0.32 0.2–0.3 155–163 36–40 - 196–213 - 14 [192] CF/PP 0.55 0.2–0.3 222–243 86–91 - 100–116 - 12–13 [192] Flax/PLA 0.40 0.5–0.7 65–115 5–8 15–75 6–8 - - [99] Graphite/PEI 0.61 0.18 1150 103 - - - - [196] Graphite/PPS 0.61 0.08 1770 131 1820 117 - - [197] * Reaction injection molding (RIM) pultrusion. Polymers 2021, 13, 180 13 of 36

Table 4. Mechanical properties of thermoset pultruded components.

Material Volume Fraction Tensile Strength, MPa Elastic Modulus, GPa GF/ 0.5–0.8 307–1320 21–59 GF/Vinylester 0.6 240 18–42 GF/ 0.5 414–790 32–40 CF/Vinylester - 1400 140–145 CF/Epoxy 0.5–0.6 1213–2200 130–180

3.5. Durability of Thermoplastic Pultruded Materials If we want pultruded thermoplastic profiles to be widely used, then we need be sure of their durability. Unfortunately, durability of both thermoset [223] and thermoplastic pul- trusion has not been studied deeply enough. Articles published are mostly related to topics other than pultrusion technology. We intend to analyze reaction injection molding and press molding in brief in order to attract scholars’ and engineers’ attention to thermoplastic pultrusion durability. Under microorganism actions, the molecular structure of a polymer can biodegrade and both physical and chemical properties can also change. Polymers can be the source of energy for the microorganisms [224]. The molecular bonds can be destroyed, as well as the composite’s properties. Biodegradation depends on crystallinity, temperature, pH of the environment, humidity, molecular weight of the polymer, various additives with enzymes, and bioorganisms [225]. Not all are biodegradable, only some, such as polyvinyl alcohol (PVA), polycaprolactone (PCL), polyester, polylactide (PLA), , ny- lon, polyhydroxybutyrate (PHB), and polyglycolide (PGA) [226,227]. Some composites are nonsusceptible to the process of biodegradation; thus, starch polymers [228–235] and fish waste [236] additives are used to accelerate the process. Composite materials based on natural fibers can be of great interest as they fully recycle through biodegradation. Moreover, can be recycled in composting conditions [224,233,237]. Re- inforcements based on wood [238], aspen [239], flax, hemp, sisal [240], cellulose [241], pineapple leaves [242], and reed [243] are typically applied. Degradation rate depends on the structure of the natural fibers, like the flexural strength of composites based on nonwoven mat decreases more than that of a woven composite [237]. Although the reaction of polymerization in thermoplastics composites is complete, the shelf life of the polymers is virtually unlimited [51–57]. The degradation of material properties may occur over time due to fatigue loads, temperature exposure, humidity, chemical reactions, radiation, etc. For example, fatigue provokes fiber failure, matrix cracking, interface debonding [244], and decrease in material strength [245]. Polymers behave differently depending on heating conditions and temperatures. Fatigue strength decreases faster at cryogenic temperature comparing with room tempera- ture [246]. Long-term aging of at a temperature below melting causes a change in glass transition temperature and strength [247,248]. During short-term aging, thermoplastic composite is sharply heated to thermal decomposition temperature; thus, rapid decrease in tensile and interlaminar shear strength is observed [249,250]. Apart from debonding and cracks, delamination and fiber failures can occur [251,252]. Freeze and thaw cycles, accompanied by cooling the material below 0 ◦C, lead to loss of flexural strength and Young’s modulus [253]. Water immersion and exposure to humidity affects tensile, compression, and flexural strength [254]. Mechanical properties depend on the temperature of the surrounding medium [254]. Typically, thermoplastic composites are studied in seawater [254–256], tap water [257,258], etc. Acid and harsh environments negatively affect the molecular bonds and mechanical characteristics of the polymers [259]. Various gases, such as air, air under pressure, and nitrogen, reduce the strength of composites [260]. Research on thermoplastic pultruded material behavior, when placed in a different harsh environment, is needed if we want wide use of profiles in marine and chemical engineering. Nuclear engineering is interested in the study of reactive radiation effects. Polymers 2021, 13, 180 14 of 36

Durability of both matrix and fiber reinforcement is to be studied as well. Experiments conducted at high and low temperatures are needed to apply the thermoplastic pultruded profiles for civil engineering in different climatic zones.

3.6. Future Trends The properties of the pultruded profiles depend greatly on the choice of raw materials. The lack of knowledge in the field of thermoplastic pultrusion becomes apparent in the choice of raw materials, as opposed to the thermoset pultrusion. To popularize the applica- tion of thermoplastic pultrusion, we need better understanding of physical phenomena taking place during the manufacturing process, and their dependence on the choice of raw materials, and clearer understanding of the influence raw materials have on mechan- ical performance and the life cycle of thermoplastic composite materials and structures. This subchapter will briefly discuss the most promising areas of research, from the authors’ point of view. In spite of availability of several studies on the influence of raw materials on mechan- ical performance of pultruded thermoplastic composites and structures, we believe that more research is necessary in order to study this question. Of special importance here are the studies of stress–strain state in composite materials under different modes and rates of loading. An in-depth research of existing and perspective additives is necessary to better understand their influence on the properties of end products and to improve mechani- cal performance and physical properties of pultruded thermoplastic profiles. Currently, only the studies by Markov [203] and Chen et al. [141] are available in this field. In addition, there is an obvious lack of studies on the influence of micro- and nanoadditives, both in thermoset pultrusion (Kuruvilla and Renukappa [261], Manjunath et al. [262]) and in the thermoplastic one (Roy et al. [263], Alam et al. [264]). Striving to reduce environmental footprint, human society demonstrates ever increas- ing interest in sustainable development and the use of natural materials, and the composite industry is no exception. Application of biocomposites is currently one of urgent research topics in thermoplastic materials [265–270]. Broad introduction of such composites into everyday life will obviously require extensive study of their properties and characteristics. While the influence of additives improving UV-aging performance and corrosion resistance of end products is mostly well understood for the thermoset pultrusions, this is not the case for the thermoplastic ones. Therefore, the real application of thermoplastic com- posites will require extensive studies of their behavior in the presence of various additives, which might be of special interest for the industrial and scientific community. In addition, of great interest for the composite community is the influence of nonbiodegradable and flame-retardant compounds on the properties of thermoplastic materials. The use of hybrid reinforcements (e.g., the simultaneous use of glass and carbon reinforcing fibers) is currently one of the hottest topics, since some loaded parts of pultruded structures may benefit from the use of different fiber types. However, there is a lack of knowledge on this issue both in thermoset [271] and, particularly, in thermoplastic pultrusion.

4. Process Modeling Manufacturing of pultruded thermoplastic composites depends on various process parameters such as preheater temperature, heated die geometry, temperature and pressure inside a heated die, cooling die temperature, pulling speed, and pulling force, and, thus, necessitates the development of mathematical models for process optimization. In addition to process parameters, the properties of prepregs, such as melting temperature, glass transition temperature, coefficient of thermal expansion, etc., should also be taken into account. All things considered, mathematical models should allow an engineer to deter- mine the degree of impregnation, temperature distribution, pulling force, etc., for complex profile geometries. These models, based on modern methods, would allow calculation of residual stresses in a composite, making it possible to predict cracking, warping, shrink- Polymers 2021, 13, 180 15 of 36

age, and other process-induced deformations. Among the published articles and books on mathematical modeling in thermoplastic pultrusion, of particular interest is the book by Suresh Advani and Murat Sozer [272]. Currently, several studies of residual stresses and strain are underway in the field of thermoset pultrusion [30,171,273–275], while the number of similar studies for thermoplastic pultrusion is significantly lower. An overview of existing mathematical models of nonreactive thermoplastic pultrusion follows. Polymers 2020, 12, x FOR PEER REVIEW 15 of 36 4.1. Impregnation 4.1. Impregnation Properties of final products depend on the fiber volume fraction. The volume frac- Properties of final products depend on the fiber volume fraction. The volume fraction tion and strength of material, in turn, depend on fiber impregnation [276] impeded by and strength of material, in turn, depend on fiber impregnation [276] impeded by the high the high viscosity of thermoplastics. Several mathematical models were developed to viscosityfind optimum of thermoplastics. manufacturing Several conditions mathemat andical to investigate models were the relationshipdeveloped to between find opti- the mumdegree manufacturing of impregnation conditions and process and to parameters. investigate Most the relationship of these models between describe the degree the nonre- of impregnationactive thermoplastic and process pultrusion parameters. with commingled Most of these yarns models and aredescribe based onthe thenonreactive following thermoplasticapproximations: pultrusion with commingled yarns and are based on the following approx- imations: • Reinforcing fibers are represented by separate groups (agglomerations) in the thermo- • Reinforcing fibers are represented by separate groups (agglomerations) in the ther- plastic melt (Figure 10); moplastic melt (Figure 10); • These groups have an elliptical or circular section; • These groups have an elliptical or circular section; • Fibers are impregnated uniformly over the bulk of the product on all sides. • Fibers are impregnated uniformly over the bulk of the product on all sides.

Figure 10. Schematic illustrationFigure of 10.yarnSchematic section and illustration consolidatio of yarnn process; section andPg—pressure consolidation from process; the void, Pg—pressure Pc—capillary from the pressure, PA—applied pressure.void, Pc—capillary pressure, PA—applied pressure.

TheThe aim aim of of these these models models is is to to determine determine the the degree degree of of impregnation impregnation at at any any moment moment ofof time time and and to to estimate estimate the the void void content content [209,213]. [209,213]. The The motion motion of of thermoplastic thermoplastic melt melt throughthrough fibers fibers is is governed governed by by Darcy Darcy law law describing describing the the flow flow of of fluid fluid through through a a porous porous mediummedium [277]: [277]: K u = − ∇p, (1) =− ∇, µ (1) wherewhere u—the—the speed ofof fluidfluid motion motion inside inside fibers, fibers,K —fiber—fiber permeability permeability tensor, tensor,µ—viscosity, μ—vis- ∇—nabla operator, and p—external pressure. cosity, ∇—nabla operator, and —external pressure. Taking into account the fiber volume fraction and local speed of fibers and thermo- Taking into account the fiber volume fraction and local speed of fibers and thermo- plastic melt, Bernet et al. [213] obtained the following expression for Darcy law: plastic melt, Bernet et al. [213] obtained the following expression for Darcy law:   K 1 − ( −1)−=V − (u∇−, u ) = − ∇p,(2) (2) f l s µ

where —fiber volume fraction, and —local speeds of thermoplastic melt and fi- where V —fiber volume fraction, u and u —local speeds of thermoplastic melt and fiber, ber, respectively.f l s respectively. As the permeability is not constant and uniform in all directions, the impregna- tion process is difficult to model. In order to calculate permeability in the direction parallel to fiber orientation, the Kozeny–Carman equation is used [211]:

∙() ∥ = , (3) ∙∙

where —fiber radius, —instantaneous fiber volume fraction depending on the pres- sure, and —the permeability constant. To determine permeability of fibers in transverse direction, the equation proposed by Gutowski et al. is used [278]:

Polymers 2021, 13, 180 16 of 36

As the permeability K is not constant and uniform in all directions, the impregnation process is difficult to model. In order to calculate permeability in the direction parallel to fiber orientation, the Kozeny–Carman equation is used [211]:

3 2   r f · 1 − Vf = Kk 2 , (3) 4·k0·Vf

where r f —fiber radius, Vf —instantaneous fiber volume fraction depending on the pressure, and k0—the permeability constant. To determine permeability of fibers in transverse direction, the equation proposed by Gutowski et al. is used [278]:

q 3 r2 Va − 1 f Vf K⊥ =   , (4) 4k Va + 1 0 Vf

where Va—maximum possible fiber volume fraction and k0—the permeability constant. When modeling the impregnation process during pultrusion, one should consider the motion of matrix along the fibers. Kim et al. [212] proposed the micromodel describing the impregnation of fibers in a transverse direction, and the macromodel describing the lengthwise flow of matrix. The macromodel was based on Darcy law and the Kozeny– Carman equation. Later, the model was supplemented with mass conservation equations in a cylindrical coordinate system (Equation (5) for matrix, and Equation (6) for fiber reinforcement): ∂   1 ∂    1 − V + 1 − V ru = 0, (5) ∂t f r ∂r f l ∂ 1 ∂   V + V ru = 0, (6) ∂t f r ∂r f s where r—matrix propagation front. Koubaa et al. [279,280] have developed a pultrusion impregnation model based on the Navier–Stokes equation, having the following expression in a cylindrical coordinate system: ∂P µ ∂  ∂u  = r z , (7) ∂z r ∂r ∂r

where z—coordinate axis coinciding with the pulling direction and uz—fluid motion speed along the longitudinal axis. The following boundary conditions are imposed: zero component of fluid motion speed outside fibers, and equality of fluid motion speed inside fibers and of the pulling speed. The model proposed by Gibson et al. [281] takes into account the capillary force. Sala and Cutolo [163] conducted numerical and experimental studies and proposed the model that uses both Newtonian and power-law relationships to predict the impregnation process outcomes. At the same time, Haffner et al. [282] developed a mathematical model that describes the microscopic flow of resin and accounts for different fiber arrangements, volume fraction of reinforcement, and impregnation time. Miller et al. [38] proposed the impregnation model for a composite material based on towpregs. The model represents a cell consisting of three filaments with two thermoplastic particles incorporated in spaces between filaments. Melting thermoplastic particles impregnate the filaments and fill the space between them. The proposed model considers the external pressure exerted by the fiber bed, capillary pressure, viscous pressure resulting from matrix motion, and springing pressure from fiber compaction. The resulting equation for impregnation time accounts for filament diameter and the size of particles, assuming them constant over the whole bulk of material. Results obtained with the analytical model are close to the experimental data, demonstrating good accuracy of the model. Subsequently, Bechtold et al. [283] proposed Polymers 2021, 13, 180 17 of 36

two different methods to model the impregnation process in the case of thermoplastic pultrusion with braided commingled yarns. Koubaa et al. [284] studied the impregnation of a single glass-fiber bundle and proposed the model based on the Young–Laplace law that takes into account the influence of capillary force. Ngo et al. [285] proposed a model of thermoplastic pultrusion with carbon fiber-reinforced prepreg that accounts for a multiscale 3D impregnation die.

4.2. Temperature Distribution The thermal model makes it possible to determine the distribution of temperatures over the heated die and the degree of crystallinity of a thermoplastic polymer. All de- veloped models rely on the heat transfer equation adapted to the pultrusion process, which results in introduction of a pulling speed term on the left side of the following equation [286]: ∂T ∂  ∂T  ρ CV = k + Q, (8) ∂x z ∂z where ρ—specific density, C—specific heat capacity, V—pulling speed, x—coordinate axis parallel to the die block axis, z—coordinate axis perpendicular to the die block axis, T—temperature, and Q—energy released during crystallization. Most polymers are amor- phous and do not form a crystal lattice [286], making it possible to disregard the Q variable in most cases. Using the model proposed by Åstroöm and Pipes [169,287], Babeau [288] conducted the experimental test and numerical simulation, and obtained results that are very close to the analytical model. Carlsson [96] proposed the following expression for the energy released during crystallization of polypropylene:

∂α Q = m H, (9) ∂t where m—mass fraction of a polymer matrix, α—degree of crystallinity, and H—theoretical ultimate heat of crystallization at 100% crystallinity. The crystallization process can be divided into two stages. The first stage is the formation of primary nuclei. The second stage is the growth of a crystal formed on the nuclei. Thus, the derivative of the degree of crystallinity can be expressed as follows [96]:

∂α (T, α) = ( f (T) + f (T)α)(1 − α), (10) ∂t 1 2

where the f1 function accounts for formation of primary nuclei and f2 accounts for further growth of the crystal. Expressions for f1 and f2 were proposed by Malkin [289]. In addition to analytical methods, scientists often use numerical methods to determine the degree of crystallinity and the heat transfer, as explained by Haffner et al. [290]. Trying to minimize modeling oscillations in the case of high pulling speed and rough mesh, Ruan et al. [291] developed a 2D thermal model of thermoplastic pultrusion. Subsequently, Yn et al. [142] utilized the finite difference method to predict temperature and reaction evolutions within the pultruded profile and to optimize process parameters while maximizing the thickness of pultruded profile. Nejhad [148] proposed and verified experimentally a numerical model dealing with thermal analysis of impregnated tows/tapes in thermoplastic pultrusion. Numerical modeling providing information on both temperature distribution within a profile during pultrusion and crystallization kinetics of the polymer was proposed by Carlsson and Astrom [292]. Ahmed et al. [293] applied the FE–NCV (finite element–nodal volume control) approach to determine the heat distribution over the heated die block and to calculate the degree of crystallinity. Aside from Ahmed, Joshi and Lam [294] used the same modeling approach to investigate crystallization in a composite based on carbon fibers and PEEK polymer (CF/PEEK) specimen. Polymers 2021, 13, 180 18 of 36

4.3. Pressure and Pulling Force Åstroöm [287] proposed a model describing the distribution of pressure over the heated die and the model of pulling force. He used the integral relationship between the pulling force and the drag over the unit area, expressed in the following form:

ftot(x) = (1 − Ω(x)) fv(x) + fc(x) + Ω(x) f f (x), (11)

where Ω(x)—the part of a composite subjected to pressure, fv(x)—viscous drag for Carreau fluid, fc(x)—compaction resistance resulting from fibers compaction in the tapered portion of the heated die block, and f f (x)—friction resistance. The model [287] uses the Carreau model and considers the nonlinear nature of ther- moplastic melt viscosity: n−1  2 2 ηa = η0 1 + (λγ) , (12)

where η0—zero-shear rate viscosity, γ—shear rate, λ—indicates the shear rate at which shear-thinning effects become significant, and the dimensionless constant n describes the degree of shear thinning. The comparison of results obtained with the analytical model and experimental data can be found in [155]. Lee et al. [286,295] proposed a numerical model to predict pressure and pulling force, as well as temperature and crystallinity. Simultaneously, a model predicting pulling resistance from the die, together with temperature and pressure distribution within a composite was developed by Astrom and Pipes [155,296]. Parasnis et al. [297] studied the influence of viscosity and shear load on the pulling force. They used a finite element model and compared calculated pulling force values with experimental data. The authors reported a discrepancy between experimental and calculated data, appearing after a certain value of pulling speed was reached. While the model predicted an exponential increase in pulling force, the experiments demonstrated that the increase in pulling force takes place until a certain pulling speed value was reached; after reaching this value, the pulling force remained unchanged. Blaurock and Michaeli proposed a method predicting pulling force and then compared it with experimental data [298]. Stavrov and Tsvirko [179] analyzed the relationship between pulling force and viscous characteristics.

4.4. Future Trends Existing impregnation, and temperature and pressure distribution models rely on a series of approximations allowing a determination of process parameters for simple profile and die shapes, and mostly for commingled yarns. However, modeling of thermoplas- tic pultrusion for complex cross-sections, towpregs, and PCT still remains an open issue. Thermoplastic pultrusion cannot boast significant progress in mathematical modeling as opposed to the thermoset pultrusion, where it widely applied, first, to model complex shaped profiles (L- and I-shaped sections [274], wind turbine blades [299,300], etc.) with complex reinforcement lay-ups, and, second, to develop algorithm for their optimization [301–317]. In recent years, a significant interest to optimization has been observed in the scientific and engineering community. Optimization tools allow engineers to solve a large number of problems, from pultrusion process optimization to optimization of raw materials for pultruded profiles, and to take full advantage of composite materials. However, in spite of certain advances in optimization, there is, still, a lack of knowledge and experience on multiobjective optimization, both in thermoplastic and thermoset pultrusion. Aside from process parameters optimization, multiobjective optimization makes it possible to optimize the geometric topology of composites [318]. Modern mathematical models should allow a solution of process optimization problems with large number of input parameters. More- over, modeling methods may help researchers investigate residual stresses in a composite, and their influence on cracking, delamination, warpage, shrinkage, and other process- induced defects [171,275]. The authors believe that expanding the use of supercomputers will bring the problems of mathematical modeling and optimization of composites to a Polymers 2021, 13, 180 19 of 36

fundamentally new level. Thus, aside from solving problems discussed earlier, the growing computational power will allow us to explain and model macrobehavior of composite materials, based on their microscale parameters.

5. Application 5.1. Pultrusion Market The pultrusion market demonstrates steady growth from year to year. According to the European Pultrusion Technology Association (EPTA) forecast [319], the pultrusion market is expected to reach the mark of €100 billion in 2022. This growth opens new oppor- tunities both for the thermoset and thermoplastic pultrusion. Thermoplastic pultrusion steadily gains popularity along with the thermoset one, although at a slower pace. For the sake of comparison, the entire thermoplastic composites market, including, aside from pultrusion, all the other thermoplastics applications as well, is expected to grow from €22.2 billion in 2020 to €31.8 in 2025, according to the report titled “Thermoplastic Com- posites Market by Resin Type (Polypropylene, Polyamide, Polyetheretherketone, Hybrid), Fiber Type (Glass, Carbon, Mineral), Product Type (SFT, LFT, CFT, GMT), End-Use Industry, and Region-Global Forecast to 2025” [320]. The main factor limiting the application of thermoplastic pultrusions is the price, i.e., the availability of thermoplastic resins, since they cost more than those used in thermoset pultrusion. This is one of the factors restraining the growth in thermoplastics applications [320]. Therefore, lower production costs, and, thus, lower final price of the manufactured products could stimulate the demand for thermo- plastic profiles, offering competition to thermoset profiles, although both reports should be considered in the context of coronavirus (SARS-CoV-2) COVID-19 pandemic situation. To illustrate, in November 2020, the Federation of Reinforced Plastics reported a 12.7% drop in production of glass-fiber-reinforced plastics in Europe, reaching the mark of 996,000 tons in 2020, which is the steepest drop since the global economic crisis of 2008–2009 [321]. According to the same report, pultrusion production volumes in 2020 plunged by 10.7%, making the pultrusion industry the least affected by the crisis, when compared to all other composite sectors.

5.2. Patents According to registered patents, one of the most common applications of thermoplastic pultrusion are thin [322–325], round, and rectangular profiles. Typically these cross-sections are utilized as wires [326,327] and their coatings [328], rods [329–331], pipes [332,333], and hollow profiles [334,335] used in the production of doors and windows [336,337], etc. Global IP Holding Co. LLC patented the method to produce parts of sandwich struc- tures [338,339] using the pultrusion process. They also patented constructions that combine both metal elements and composite materials [340,341]. Various combinations of fiber struc- tures are used; for example, some of the layers are made of unidirectional fibers [342], while the others are made with transversely oriented fibers [343], fabrics [344,345], or long-fiber thermoplastics (LFTs) [346]. Pultruded profiles are widely used in railway construction. A group of engineers from Pultrusion Technique Inc. developed a method for the produc- tion of rail clamps, providing the benefit of excellent corrosion resistance [347], as compared to their iron counterparts. In addition, they patented wavy profiles [348] and elements with asymmetrical shapes [349,350].

5.3. Current Applications of Thermoplastic Pultruded Profiles Pultruded thermoplastic profiles effectively combine properties of thermoplastic com- posites with advantages of pultrusion as a manufacturing process. Offering improved toughness and fire resistance, thermoplastic composites find application in many industries. Pultruded thermoplastic composites have found wide application in aerospace and [67,69] aviation [64,66] engineering. For example, landing gear doors made of thermoplastic composites have lower weight, compared to their aluminum counterparts, are weldable, and can be recycled, as opposed to those made of thermoset composites [68,351]. Thermo- Polymers 2021, 13, 180 20 of 36

plastic composites can be used to manufacture airplane flooring [68], ice protection panels protecting the fuselage [351], various interior elements [351], rivets for fastening [15,65], aircraft wings [68], radomes [68], and flaps [68]. Aside from aviation, thermoplastic compos- ites are widely used in the automotive industry [59,60,62,63]. The ability of thermoplastics allows fabrication of various complex shape parts, such as dashboard - ers [61], body structures [61], bumpers [58,61,352], wheel rims [353], and seat structures [61] (commonly produced from long-fiber thermoplastics (LFT) [354]), etc. In civil engineer- ing [70,71], thermoplastic composites are used to manufacture airfoils for wind turbine blades [355], pipes [84,85], rebars [86,87] and rods [88–91], reinforcement for concrete struc- tures [86], window profiles [83], elements of walls [72], flooring [72], exterior siding [72], and roofing systems [72,73]. In addition, composite poles used in powerline-supporting structures for energy grids are often produced by thermoplastic pultrusion [74]. Moreover, pultruded elements can be used in restoration of deteriorated structures and rehabilitation projects [75]. Aside from these applications, there is a demand from the marine [76–79,356] and oil/gas [80] sectors. In view of product recycling capabilities, thermoplastic pultrusion is the process of choice for production of semiproducts for LFT [357–363] and cork and pellet composites [364–368]. Pultrusion can produce prepregs for constant size LFT with specified length of fibers and precisely maintained fiber volume fraction. In spite of a steady growth in application of thermoplastic pultrusions in auxiliary elements of structures in the last few years, there are no published articles, patents, or news on the application of such profiles in the design and construction of full-scale bearing structures, such bridges, cooling towers, etc. This can be explained by the lack of knowledge on the behavior of these types of structures, which are produced of thermoplastic pultruded elements. Scientists and engineers have yet to investigate the strength, buckling, creep, fatigue, and durability aspects of thermoplastic pultruded profiles as applied to the full- scale structures. Advances and experience in implementation of such structures are almost completely lacking, as there are no relevant design codes. European [369–371] and US [372] design codes regulating the design of pultruded structures deal only with thermoset profiles, or have no clear mention of pultrusion type. Therefore, these design codes can only be freely applied in the design of thermoset pultruded structures, as they were specifically developed for these types of profiles. The peculiarities of thermoset and thermoplastic pultrusion processes will undoubtedly impose certain limitations on the behavior of profiles under a particular loading mode. The behavior of thermoplastic and thermoset profiles under the same load may differ significantly. Thus, to account for specifics of thermoplastic profiles, existing structural design codes should be revised or rebuild anew. This will require extensive experimental investigations, the results of which will be used as a basis for future design codes.

5.4. Future Trends Advanced studies mentioned in this subchapter show that pultruded thermoplastic profiles can be applied both in traditional areas mentioned previously, and in some, at first sight, nonconventional ones. The authors believe it is important to draw the attention of the composites research community to these perspective fields of research. Application of green technologies and materials in manufacturing is a hot topic both in the composite community and in other industries [373]. Multiple studies conducted in the last few years demonstrate that human society should do its best to thoughtfully use and recycle the products of its activity, and to minimize its carbon footprint. Recent studies in recycling of thermoplastic composites demonstrate the growing interest to this field. There are various mechanical, chemical, and thermal approaches to composites recycling. The main problem associated with the application of recycled composites lies in a degradation of mechanical properties of recycled fibers to be used in newly produced composites [374,375]. On the other hand, striving to minimize carbon footprint, researchers opened a new perspective on the application of polyethylene terephthalate (PET), typi- Polymers 2021, 13, 180 21 of 36

used in worldwide packaging, as a raw material for thermoplastic pultrusion [187]. Thus, any investigations aimed at application of recycled and natural raw materials in thermoplastic pultrusion will have a good perspective. The application of thermoplastic polymers in composites holds considerable promise for the use of welded joints; however, the performance of such joints is yet poorly under- stood and will require the analysis on a case-by-case basis. In spite of the large number of publications in a field of thermoplastics welding in general [376–379], there are no studies on the welding of pultruded thermoplastic composites in particular. There are a few studies reporting on the medical application of thermoplastic pul- truded profiles. Tanimoto et al. [380,381] manufactured and investigated the properties of pultruded glass-fiber-reinforced wiring for orthodontic applications. Engi- neers from Fraunhofer Institute for Production Technology developed the process allowing a pultrusion of thermoplastic elements as small as 1 mm in diameter, which can be used in medical applications [382,383]. Authors report the excellent compatibility with magnetic resonance imaging (MRI) techniques and good post formability. Thermoset pultrusion was used to fabricate guidewire appliances that were mechanically tested along with in vivo ex- periments on animals [384]. In addition, the appliance compatible with magnetic resonance (MR) was pultruded and tested in various MR-guided cases aimed to study the behavior of arteries [385]. Recently, an experimental study [386] demonstrated the feasibility of thermoset micropultrusion of 280 µm-diameter carbon fiber elements, and proposed the use of thermoplastic matrices as a recommendation for further research. Invented in ETZ Zurich, continuous lattice fabrication (CLF) is a new additive manu- facturing (AM) technique making it possible to print thermoplastic fiber reinforced poly- mers (FRPs) in three-dimensional space at any imaginary trajectory, with the help of robotic arms [387–389]. This approach combines both extrusion and pultrusion. Introduction of AM or robotic fabrication techniques into thermoplastic pultrusion manufacturing would definitely broaden the perspectives for thermoplastic pultrusion. Meanwhile, a novel technique proposed by the Institute of Plastics Processing at RWTH Aachen University presents hybrid pultruded profiles combining both a thermoset core and thermoplastic top layer, combining the advantages of both processes [390]. Combination of thermoplastic and thermoset pultrusions may certainly result in more efficient structural components, thereby exploiting the advantages of both manufacturing techniques. As the human society demonstrates the increasing interest in colonization of Moon and Mars, there will be a large demand for space transport technologies and to produce various structures for space stations, power generation platforms [391], and other facilities necessary to settle on other planets. There are no considerable obstacles to shipping a pul- trusion machine into space [3] to utilize the advantages of thermoplastic polymers [392,393]. British company Magna Parva, specializing in space research, plans to use pultrusion in space where no kind of production has been done before [394]. Typically, pultruded profiles are supposed to have straight shapes; however, pultru- sion of nonlinear profiles can be accomplished as well. Curved pultruded profiles can be used to minimize the excessive deflection of structures, to implement components of complex shapes, or to impart individuality to architectural forms. Currently, fabrication of curved pultruded profiles is actively investigated in thermoset pultrusion [395–397]. However, similar studies in thermoplastic pultrusion are very limited [398], and, therefore, this issue requires further investigation. According to the studies on urban planning and development, over 70% of world pop- ulation will live in cities by 2025. Steadily growing migration from countryside to the cities forces telecom companies to search for solutions to today’s technology challenges. Large megapolises require effective and innovative information services and data transmission facilities. Offering data rates of 10–20 Gbps, 5G may be a solution to these problems [399]. Pultruded profiles, being transparent to radio frequency signals, are perfectly suited for use in the growing 5G network infrastructure around the globe [399]. Polymers 2021, 13, 180 22 of 36

In addition, the growing interest in smart polymers, materials able to change their physical and chemical properties under the influence of various external factors (pH, temperature, UV light, etc.) [400], may also apply to thermoplastic pultrusion. Of special interest are shape memory polymers, a subset of smart polymers, which are able to recover their shape under the influence of certain external factors [401,402]. These materials with their unique properties can have various applications —in medicine [403] and self-healing systems [404], and in aerospace [405], electronic [403] and civil [403,406] engineering.

6. Conclusions This study reviews the state-of-the-art in thermoplastic pultrusion. We discussed the distinctive features of the process, materials used, patents registered, properties of pultruded profiles, industrial market situation, and applications of thermoplastic pultruded profiles. Application of thermoplastic polymers in pultrusion instead of thermoset ones makes it possible to improve the impact strength of structures, and offers the advantages of recycling, indefinitely long storage of source material, and application of welded joints of composite profiles. However, the limited number of studies in the field of pultruded thermoplastic composites makes it difficult to unveil the full potential of thermoplastic pultrusion. Trying to answer the question of the huge industrial, scientific, and experience gap existing between thermoset and thermoplastic pultrusion, we were able to develop recommendations on further research in the application of composite structures in general, and pultruded thermoplastic profiles in particular. We also recommend the research areas necessary to broaden the field of thermoplastic profiles application in order to obtain the knowledge sufficient for understanding the complex mechanics of thermoplastic composites, which is necessary to design complex critical structures currently built of thermoset profiles. It must be noted that this review, being the first of its kind (as no review papers on thermoplastic pultrusion were published earlier), discusses only general questions and does not probe deeper into specific aspects of thermoplastic pultrusion and the materials produced. However, as there is an urgent need for such studies, in the near future we can expect publication of separate review papers concerning specific subtopics of thermoplastic pultrusion, such as, for instance, additives, structural design, durability of pultruded thermoplastic elements, process-induced shape distortions, biocompatibility, and natural materials, among others.

Author Contributions: Conceptualization, A.S., I.A., A.V., and K.M.; methodology, A.S., A.V., and K.M.; writing—original draft preparation, K.M., A.V., and A.S.; writing—review and edit- ing, K.M., A.V., and A.S.; visualization, K.M.; supervision, A.S.; project administration, A.S.; funding acquisition, I.A. All authors have read and agreed to the published version of the manuscript. Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Conflicts of Interest: The authors declare no conflict of interest.

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